Continuous process for scrubbing sulfur dioxide from a gas stream

ABSTRACT

An input gas stream containing SO2 is continuously treated in order to remove a substantial portion of the SO2 therefrom by the steps of: (a) scrubbing the input gas stream with an aqueous absorbent containing an alkaline reagent; (b) treating the resulting rich absorbent stream with a reducing agent at conditions selected to convert the sulfite compound contained therein to the corresponding thiosulfate compound; (c) reacting the resulting thiosulfate compound with carbon monixide at reduction conditions selected to produce the corresponding sulfide compound; (d) stripping hydrogan sulfide from the resulting solution to form a regenerated aqueous absorbent stream; and, thereafter, (e) passing at least a portion of the regenerated absorbent stream to the scrubbing step.

United States Patent [191 Urban [451 *Apr. 17,1973

[ CONTINUOUS PROCESS FOR SCRUBBING SULFUR DIOXIDE FROM A GAS STREAMPeter Urban, Northbrook, Ill.

[73] Assignee: Universal Oil Products Company,

Des Plaines, Ill.

[75] Inventor:

[*1 Notice: The portion of the term of this patent subsequent to Apr. 6,1988,

has been disclaimed.

[22] Filed: Mar. 26, 1971 211 Appl. No.: 128,428

Related US. Application Data [63] Continuation-impart of Ser. No. 9,894,Feb. 9, 1970,

Pat. No. 3,574,097. w

[52] US. Cl. ..423/242, 423/541 [51] Int. Cl. ..C01b 17/16 [58] "Fieldof Search ..23/2, 181, 225;

['5 6] References Cited UNITED STATES PATENTS 2/1970 Urban .2312 R XPrimary Examiner-Oscar R. Vertiz Assistant Examiner-George O. PetersAttorney-James R. Hoatson, Jr. and Thomas K. Mc- Bride 57 ABSTRACT.produce the corresponding sulfide compound; (d)

stripping hydrogan sulfide from the resulting solution to form aregenerated aqueous absorbent stream; and, thereafter, (e) passing atleast a portion of the regenerated absorbent stream to the scrubbingstep.

21 Claims, 1 Drawing Figure CONTINUOUS PROCESS FOR SCRUBBING SULFURDIOXIDE FROM A GAS STREAM CROSS REFERENCES TO RELATED APPLICATIONS Thisapplication is a continuation-in-part of my prior, copending applicationentitled TREATING A WATER STREAM CONTAINING A WATER-SOLU- BLE SULFITECOMPOUND which was filed Feb. 9, 1970 and assigned Ser. No. 9,894 nowUS. Pat. No. 3,574,097.

The subject of the present invention is a novel, continuous process forthe selective removal of SO from a gas stream containing same. Moreprecisely, the present invention involves an SO -scrubbing step operatedwith an aqueous absorbent containing an alkaline reagent coupled with aunique regeneration procedure which comprises: a preliminary treatmentstep wherein sulfite compound contained in the rich scrubbing solutionare converted to the corresponding thiosulfate compound, a primaryreduction step wherein the resulting thiosulfate compounds are convertedto the corresponding sulfide compound,-and a stripping step wherein theabsorbent solution is regenerated with liberation of hydrogen sulfide.In one important aspect, the present invention has to do with ascrubbing process which is operated with highly efficient aqueousabsorbent comprising ammonium hydroxide and/or carbonate wherein therich scrubbing solution withdrawn from the scrubbing step containedammonium sulfite and/or ammonium bisulfite, wherein the preliminarytreatment step involves selective conversion of the ammonium sulfiteand/or ammonium bisulfite to ammonium thiosulfate, wherein the primaryreduction step involves selective conversion of ammonium thiosulfate toammonium sulfide and/or hydrosulfide and wherein H 8 is stripped fromthe resulting solution with CO to form the regenerated absorbent.

A major problem encountered in many areas of industry today isassociated with the production of waste gas streams containing sulfurdioxide. The problem essentially involves the disposal of these wastegas streams without causing substantial air pollution. This problem isan extremely complex one because of the wide variety of industrialsources that emit these sulfur dioxide-containing gas streams. One ofthe more common sources is associated with the combustion ofsulfur-containing fuels in boilers, internal combustion engines, heatingunits, etc., to produce flue or stack gas streams containing significantamounts of sulfur dioxide. Similarly, waste gas streams of this type aregenerally produced by other industrial processes such as the smelting ofsulfur-beating ores, the refining of coke, the production of sulfur in aClaus process, the production of paper via a wood pulping process andthe like industrial processes. It is well known that the indiscriminatedischarge of these gas streams into the atmosphere results in asubstantial air pollution problem because the sulfur dioxide hasextremely detrimental effects on animal and plant life. In addition, thedischarge of these gas streams into the atmosphere constitutes a wasteof a valuable material because the sulfur contained in same is anindustrial commodity. Many processes have been proposed for removal ofsulfur dioxide from these gas streams. A larger percentage of theseproposed removal procedures involve contacting the sulfur dioxidecontaining gas stream with an aqueous absorbent stream which typicallycontain materials which chemically or physically react with the sulfurdioxide in order to absorb same into the liquid solution. A widelystudied procedure involves the use of a solution ofan alkaline reagentsuch as ammonium hydroxide or carbonate, one of the alkali or alkalineearth metal salts such as sodium hydroxide or carbonate, potassiumhydroxide or carbonate and the like alkaline reagents, to produce a richabsorbent stream containing the corresponding sulfite and/or bisulfitecompound; For example, the use of our aqueous absorbent containingsodium bicarbonate and carbonate to form a rich scrubbing solutioncontaining sodium sulfite and bisulfite.

v Although the simple concept of the scrubbing S0 from the gas streamcontaining same with an aqueous absorbent containing an alkaline reagenthas many advantages associated with it such as simplicity, higheffectiveness and versatility, wide spread adoption of this type ofsolution to the S0 pollution problem has been inhibited by the lack of aregeneration procedure for the rich absorbent stream which cancontinuously regenerate the rich absorbent stream by selectively andeconomically converting the absorbed S0 to a conveniently handled andsaleable product in a highly selected manner. That is, it is requiredthat the regeneration procedure enable the operation of the scrubbingsystem in a closed-loop scrubber with respect to the absorbent. Inparticular, it is required that an acceptable regeneration procedurehave the capability of not only continuously producing a regeneratedabsorbent stream but also minimizing undesired by-products so as toprevent the buildup of undesired intractable, difficultly removedingredients in the absorbent stream once the system is operated in aclosed-loop fashion. The by-product that is of the greatest concern inthis regard is sulfate for example, in a system using an aqueoussolution of ammonium hydroxide or ammonium carbonate as the absorbent,ammonium sulfate and bisulfate salts once formed in the system canpresent serious problems if special means are not provided to remove itor its production is not suppressed. Specifically, these salts can buildup until finely divided solids are formed which then can causecorrosion, erosion and fouling problems.

One solution that has been proposed to the problem of regenerating therich absorbent streams of the types discussed above is the use of thesuitable reducing agent to react with the sulfite compounds containedtherein in order to selectively produce elemental sulfur and/or thecorresponding sulfide compound. However,

despite stringent precautions, when common reducing agents such ashydrogen, a suitable sulfide compound, or carbon monoxide are used in anattempt todirectly reduce these sulfite compounds to elemental sulfur orthe corresponding sulfide compounds, undesired sulfate compounds areformed in unacceptable amounts. These sulfate compounds are believed tobe caused by the sulfite compounds undergoing auto-oxidation-reductionat the conditions necssary for direct reduction.

The problem addressed by the present invention is, therefore, to providea flue gas scrubbing system which I loop fashion and comprising a wetscrubbing step coupled with a novel regeneration procedure which enablesthe recovery of hydrogen sulfide in high yield, minimizes undesiredsulfate by-products from the regeneration section and produces aregenerated absorbent stream which is of a relatively low total sulfurcontent and, consequently, possesses a high capacity for S removal. Theconcept of the present invention is based on my finding that the sulfitecompound contained in the rich scrubbing solution withdrawn from thescrubbing step can be easily converted at relatively low severityconditions to the corresponding thiosulfate compound in the highlyselective manner without forming any substantially amounts of undesired,intractable sulfate compounds. Coupled with the finding is my additionalobservation that the thiosulfate compound can be reduced by carbonmonoxidein a highly selective, economic and efficient manner to form thecorresponding sulfide compound from which hydrogen sulfide can be easilyrecovered. Thus the central point of the present process involves usinga conventional scrubbing procedure coupled with a regeneration procedurewherein thiosulfate is used as an intermediate in a multi-step operationdesigned to convert the sulfite compound contained in the rich absorbentstream to hydrogen sulfide, rather than an attempt to directly reducethe sulfite compound to sulfide in a single step operation. Thissulfite-to-thiosulfate-to-sulfide route provides a regenerationprocedure which facilitates careful control of by-product formationduring the regeneration operation and enables the production of aregenerated absorbent stream which can be directly recycled to thescrubbing step, thereby allowing the system to be operated in the closedloop fashion with respect to the absorbent stream.

It is accordingly, an object of the present invention to provide asimple, effective, efficient, and selective procedure for treating a gasstream containing SO which process can selectively produce hydrogensulfide and operate with a continuous closed-loop circuit of absorbentbetween the scrubbing section and the regeneration section. Anotherobject is to minimize the amount of undesired, intractable by-productsformed in the regeneration section of such a procedure. Another objectis to provide a regeneration procedure for an .SO,-scrubbing step thatmaximizes the sulfur differential across the regeneration procedure,thereby increasing the capacity and efficiency of the regeneratedabsorbent.

In brief summary, one embodiment of the present invention is a processfor treating an input gas stream containing SO, in order to continuouslyremove a substantial portion of the SO, therefrom. The first step is ascrubbing step wherein the input gas stream is contacted in a suitableliquid-gas contacting zone with an aqueous absorbent containing analkaline reagent at scrubbing. condition selected to result in a treatedgas stream containing a substantially reduced amount of S0 and in aneffluent water stream containing a watersoluble sulfite compound. Thenext step is a'preliminary treatment step which involves contacting atleast a portion of the effluent stream from the scrubbing step with areducing agent, selected from the group consisting of finely dividedsulfur, a poly'sulfide compound, a water-soluble sulfide compound andmixtures thereof, at thiosulfate production conditions selected to forman effluent stream containing a thiosulfate compound. Thereafter, theeffluent stream from the preliminary treatment step is reacted withcarbon monoxide in the primary reduction step at reduction conditionsselected to produce a sulfide-containing aqueous effluent stream. Thenext step is a stripping step wherein hydrogen sulfide is stripped fromthe aqueous effluent stream from the primary reduction step to produce ahydrogen sulfide-containing overhead stream and regenerated aqueousabsorbent stream. In the final step, at least a portion of the resultingregenerated aqueous absorbent stream is passed to the scrubbing step,thereby providing a closed-loop flow circuit of ab sorbent.

In another embodiment, the invention is a process as outlined about inthe first embodiment wherein the al kaline reagent utilized in theaqueous absorbent stream is selected from the group consisting of thesalts of ammonia, the alkali metals and the alkaline earth metals whichhydrolize in water to form a basic solution for example, the hydroxideand carbonate salts. I

In a more specific embodiment, the present invention is a process fortreating-a gas stream containing S0 in order to remove a substantialportion of the S0 therefrom. The first step in this embodiment involvesa scrubbing step in which the input gas stream is contacted with anaqueous absorbent containing ammonium carbonate or bicarbonate atscrubbing conditions selected to form a treated gas stream containing asubstantially reduced amount of S0 and an effluent water streamcontaining ammonium sulfite or bisulfite. At least a portion of theeffluent water stream from the scrubbing step is thereafter contacted ina preliminary treatment step, with a reducing agent, selected from thegroup consisting of finely divided sulfur, a polysulfide, awater-soluble sulfide compound and mixtures thereof, at thiosulfateproduction conditions selected to result in an effluent streamcontaining ammonium thiosulfate. The primary reduction step theninvolves reacting the effluent stream from the preliminary treatmentstep with carbon monoxide at reduction condi tions selected to producean aqueous effluent stream containing ammonium sulfide or hydrosulfide.The aqueous effluent stream from this primary reduction step isthereafter subjected to contact with a carbon dioxide-containingstripping gas at stripping conditions effective to produce a regeneratedaqueous absorbent stream containing ammonium bicarbonate or carbonateand an overhead gaseous stream containing hydrogen sulfide. At least aportion of the resulting regenerated absorbent stream is then in thefinal step, passed to the scrubbing step, thereby providing aclosed-loop fiow circuit absorbent.

lnanother embodiment, the present invention is a process as describedabove in the last embodiment wherein the reducing agent utilized in thepreliminary treatment step is hydrogen sulfide and wherein at least aportion of the overhead stream produced in the stripping step is passedto the preliminary treatment step in order to supply a portion of thehydrogen sulfide reactant utilized therein.

Yet another embodiment of the present invention involves a process asoutlined above in the first embodiment wherein the primary reductionstep is performed in the presence of a catalyst comprising a metalliccomponent selected from the group consisting of the transition elementsof groups VI and VIII of the Periodic Table, and compounds thereof,combined with a porous carrier material.

Other objects and embodiments of the present invention are hereinafterdisclosed in the following detailed discussion of the input streams, thepreferred conditions, the preferred reactants, the output streams andmechanics associated with each of the essential and preferred steps ofthe present invention.

The starting point for the subject process is a scrubbing step whereinan input gas stream containing S is contacted in a suitable gas-liquidcontacting means with an aqueous absorbent containing an alkalinereagent. As previously explained, the input gas stream passed to thisstep is typically a flue or stack gas. For example, a typical stack gasstream containing about I to about 10% 0 about 5 to 15 percent or moreCO about 3 to about percent or more H O, about 0.01 to about 1 percentor more S0 In many cases, the input gas stream will also contain carbonmonoxide, oxide of nitrogen, entrained fly ash and the other well-knowningredients for flue gas streams. The amount of S0 contained in thisinput gas stream can vary over a wide range; namely from about 0.01 toabout 1 mole percent or more, with a more typical amount being about0.05 to about 0.5 mole percent. In many cases, this input gas stream isavailable at a relatively high temperature of about 200 to about 500F.or more, and since it is preferred that the temperature of the input gasstream be at a relatively low level because this increases the capacityof the absorbent solution, it is often advantageous to cool the inputgas stream by any suitable means such as by presaturating it with waterunder adibatic conditions.

The aqueous absorbent utilized in this scrubbing step is generallycharacterized as an aqueous solution of a suitable alkaline reagentwhich reacts with waterto give a basic solution such as ammoniumhydroxide, ammonium carbonate and bicarbonate, the alkali metalhydroxides, the alkali metal carbonates and bicarbonates and thewater-soluble alkaline earth metal hydroxide, carbonates andbicarbonates, and the like alkaline reagents. Of the alkali metalreagents, sodium hydroxide, sodium carbonate, sodium bicarbonate,potassium hydroxide, potassium carbonate, potassium bicarbonate areparticularly preferred. In most cases, excellent results are obtainedwhen the alkaline reagent is ammonium hydroxide or ammonium carbonate orammonium bicarbonate. It is to be noted that it is within the purview ofthe present invention to use a mixture of the alkaline reagentspreviously mentioned. Since it is also contemplated that the scrubbingstep can be operated with the absorbent continuously cycling around thescrubbing means, it is possible that absorbent could accumulatesubstantial amounts of sulfite and bisulfite compounds. In this lastcase, only a small portion of the rich effluent stream from thescrubbing step would be sent to the regeneration section of the process,and the major portion of the rich absorbent would be commingled with theregenerated absorbent stream and recycled to the scrubbing step.

The amount of alkaline reagent contained in the scrubbing solution issubject to some choice depending upon the specific requirementsassociated with the particular gas stream being treated; ordinarly,acceptable results are obtained when the alkaline reagent comprisesabout 1 to about wt. percent of the absorbent solution, and morepreferably to l to about 15 wt. percent. Of course, absorbent solutionscontaining an amount of the alkaline reagent up to the solubility limitof the particular alkaline reagent selected at the condi tionsmaintained in the scrubbing step can be utilized if desired. In the casewhere the absorbent is continuously cycled around the scrubbing step andonly a drag stream drawn off for regeneration, the total amount of thealkaline reagent contained in the solution (i.e. fresh and spent) canreach rather high levels; for example, it can easily constitute 30 to 50wt. percent of the absorbent solutions.

This scrubbing step can be carried out in a conventional scrubbing zonein any suitable manner including multiple stages. The scrubbing solutioncan be passed into the scrubbing zone in either upward or downward flowand the input gas stream can be simultaneously introduced into thescrubbing zone in concurrent flow relative to the scrubbing solution. Aparticularly preferred procedure involves downward flow of the scrubbingsolution with countercurrent flow of the gas stream which is to betreated. The scrubbing zone is preferably a conventional gas-liquidcontacting zone containing suitable means for effecting intimate contactbetween a descending liquid stream and an ascending gas stream. Suitablecontacting means include bubble trays, baffles, and any of the variouspacking materials known to those skilled in the art. In thiscountercurrent mode of operation, a treated gas stream is withdrawn fromthe upper region of the scrubbing zone and a rich absorbent solution iswithdrawn from the lower region thereof. For the class of alkalinereagents of concern here, the rich absorbent solution will containsubstantial amounts of a water-soluble sulfite compound such as ammoniumsulfite and/or bisulfite, sodium sulfite and/or bisulfite and the like.As indicated previously, according to one mode of operation of thescrubbing step, only a drag stream from the rich absorbent withdrawnfrom the step is sent to the regeneration section of the process; therest is cycled around the scrubbing step. The drag stream is ordinarilywithdrawn at a rate at least sufficient to continuously remove the netsulfur taken up in the scrubbing step.

This scrubbing step is generally conducted under conventional scrubbingconditions which are selected on the basis of the characteristics of thespecific alkaline reagent utilized, the sulfur dioxide content of theinput gas stream, the portion of the sulfur dioxide that is to beremoved in the scrubbing step, and the physical properties of thescrubbing zone. Ordinarily, the scrubbing step is preferably operated ata relatively low temperature of about 10 to C., a relatively low ofabout 100:1 to about 10,000:l and a pH of about 4 to 7 or more. When theinput gas stream is a flue or stack gas stream, means must ordinarily beprovided for cooling the input gas stream to a relatively lowtemperature before it is introduced into the scrubbing step. Likewise,since the typical operation of the scrubbing step involves the handlingof large volumes of gas containing only a relatively small amount ofsulfur dioxide, it is preferred that the pressure drop through thescrubbing zone be held to a minimum so as to avoid the necessityofcompressing large volumes of gas to overcome the pressure drop withinthe scrubbing zone.

Following the scrubbing step, the next step of the present process isthe preliminary treatment step and it involves the conversion, in thehighly selective manner, of the sulfite or bisulfite compound containedin at least a portion of the effluent water stream withdrawn from thescrubbing step, to the corresponding thiosulfate compound. Ordinarilythe sulfite or bisulfite compound is contained in the feed stream tothis step in an amount of about 0.01 wt. percent, calculated on anequivalent sulfur basis, up to the solubility limit of the particularsulfite compound in water at the conditions utilized in the scrubbingstep; for example, the feed stream to this step can contain about l toabout 20 wt. percent sulfur as ammonium sulfite and/or bisulfite.

size of about 10 to about 250 microns, with best results obtained withparticles of about 25 to about 100 microns. Typically, it is a goodpractice to introduce the sulfur into this step via a water streamcontaining a slurry of finely divided sulfur in an amount of about 1 toabout 75 wt. percent thereof, although many other suitable means forinjecting finely divided solid particles can be utilized if desired. Inthis first mode of operation, it is preferred to also utilize a wettingagent in the reaction mixture in order to facilitate good contact of theelemental sulfur with the sulfite compound.

Suitable wetting agents are: the salts of alkyl aryl sulfonates such asthe sodium salt of dodecylbenzene sulfonate; sulfonated fatty acidesters; C to C alkyl sulfates; C to C alkyl sulfonates; alkylpolyoxyethylene alcohols; ethylene oxide condensations products of alkylphenols; quaternary ammonium salts such as octadecyldimethylbenzylammonium chloride and the like wetting agents. The wetting agent ispreferably utilized in a relatively small amount corresponding to about0.01 to about 1 wt. percent of the sulfite compound that is reacted. Theamount of elemental sulfur utilized in this first mode of operation ofthe first step should be sufficient to supply one atom of sulfur permolecule of sulfite compound contained in the input water stream, withthe preferred amount corresponding to about one to about three atoms ofsulfur per mole of sulfite compound.

In a second mode of operation for this preliminary treatment step, thereducing agent is a polysulfide compound. Suitable polysulfide compoundsinclude the ammonium, alkali metal, and alkaline earth polysulfides.Best results are ordinarily obtained with ammonium or sodiumpolysulfide. The polysulfide compound is ordinarily charged to thisfirst step in the form of an aqueous sdlu tion containing about 1 toabout 50 wt. percent of the polysulfide compound. It is to be noted that'wheiiThe reducing agentisi'polysulfide compound, no wetting agent isnecessary in order to achieve good contact with the sulfite compound.The amount of the polysulfide compound charged to this step ispreferably sufficient to provide at least the stoichiometric amountnecessary for the reaction between it and the sulfite compound toproduce the corresponding thiosulfate compound. In the typical casewhere the polysulfite compound contains four atoms of elemental sulfurand one atom of sulfide (e.g. (NI-1.0 8 the stoichiometric amount is 1/6moles of polysulfide per mole of sulfite compound, with a preferredvalue being about one-fourth to about three-fourthsor more moles ofpolysulfide per mole of sulfite compound.

In a third mode of operation of this preliminary treatment step, thereducing agent is a water-soluble sulfide compound. Suitablewater-soluble sulfide compounds are hydorgen sulfide, ammonium sulfide,ammonium hydrosulfide and thesulfides and hydrosulfides salts of thealkali and alkaline earth metals. Best results are ordinarily obtainedin this mode of operation of this step when the sulfide reactant ishydrogen sulfide or ammonium hydrosulfide. The amount of this sulfidereactant utilized in this step is at least sufficient to provide 0.5moles of sulfide compound per mole of sulfite compound contained in theinput water stream, with best results obtained at a mole ratiocorresponding to about 0.6 to about 1.5 or more. Likewise, in this thirdmode of operation, good results are ordinarily obtained when the pH ofthe input water stream is in the range of 4 to about 7.

Conditions utilized in this preliminary treatment step can be generallydescribed as thiosulfate production conditions and comprise: atemperature of about 20 to about C., a pressure sufficient to maintainthe input water stream in the liquid phase and a contact timecorresponding to about 0.05 to l or more hours. In general, the contacttime necessary for the desired reaction is a function of the reducingagent utilized, with relatively short contact times of about 1 to 5minutes being sufficient in the case where the reducing agent is asulfide or a polysulfide compound. The other two reducing agents requirea relatively longer contact time ranging up to about 0.1 to about 1hour. Considering all of the factors involved in the operation of thispreliminary treatment step, best results are ordinarily obtained whenthe reducing agent is hydrogen sulfide or a polysulfide compound,particularly sodium or ammonium polysulfide.

Following this preliminary treatment step, an aqueous effluent streamcontaining relatively large amounts of a thiosulfate compound iswithdrawn therefrom and passed to the primary reduction step of thepresent process wherein thiosulfate is reacted with carbon monoxide atreduction conditions selected to produce the corresponding sulfidecompound.

The carbon monoxide stream for use herein may be obtained from anysuitable source or may be prepared in any suitable manner. An acceptablecarbon monoxide stream is obtained by the partial oxidation of organicmaterials, and particularly carbon at high temperature with oxygen, airor steam. Likewise, a carbon monoxide stream suitable for use herein canbe prepared by the reduction of carbon dioxide by hydrogen, carbon orcertain metals at high temperatures. For example, a gas streamcontaining about 40 percent carbon monoxide is easily prepared byblowing steam through a bed of coal at an elevated temperature. Anothersuitable carbon monoxide-containing stream is obtained by simultaneouslyblowing air and steam through a bed of red hot coal to produce a gasstream containing about 30 percent carbon monoxide. In addition, blastfurnace gases resulting from the reduction of iron oxide by red hot cokecan be utilized to supply the necessary carbon monoxide stream ifdesired. Yet another source of a suitable carbon monoxide stream is astream prepared by passing carbon dioxide and oxygen through charcoal orcoke at a temperature greater than about 1,000C. in order to decomposethe CO to CO. Regardless of the source of the carbon monoxide, it ispreferably used herein in an amount sufficient to provide a mole ratioof carbon 'monoxide to thiosulfate compound of at least 4:1, with bestresults obtained at a mole ratio of about 5:1 to :1 or more. 1 haveobserved that the amount of sulfide formed increases with higher moleratios of carbon monoxide to thiosulfate.

This primary reduction step can be carried out, if desired, without theuse of a catalyst; however, in many cases, it is advantageous to use acatalyst for this reaction. Based on my investigations 1 have determinedthat improved results are obtained in this step when the reaction zonecontains materials such as particles of charcoal, and particles ofactivated carbon. Particularly good results are obtained with a catalystcomprising a metallic component selected from the group consisting ofthe transition metals of groups V1 and VIII such as chromium,molybdenum, tungsten, iron, cobalt, nickel, platinum, palladium, etc.From this, 1 have concluded that preferred catalysts for the desiredreduction reaction comprise a combination of a group VI or a group VIIItransition metal component with a suitable porous support such asalumina or activated carbon. Particularly preferred embodiments of thepresent step involve the use of catalysts in which the metalliccomponent is present in the form of a metallic sulfide such as cobaltsulfide, or molybdenum sulfide, or tungsten sulfide combined with acarrier material. The preferred carrier materials are activated carbonssuch as those commercially available under the trade names of Norite,Nuchar, Darco and other similar products. In addition, otherconventional natural or synthetic highly porous inorganic carriermaterials may be used as the support for the metallic component such asalumina, silica, silica-alumina, etc. Best results are ordinarilyobtained with a catalyst comprising cobalt or molybdenum or tungstensulfide combined with relatively small particles of activated carbon.Excellent results have been obtained with 10 to 12 mesh activated carbonparticles containing about 5 wt. percent of cobalt sulfide. In general,the amount of the metallic' component utilized in the catalyst should besufficient to comprise about 0.1 to about 50 percent thereof, calculatedon a metallic sulfide basis. These catalysts can be prepared accordingto any of the conventional procedures for combining a metallic componentwith a carrier material, with an impregnation procedure with a soluble,decomposable compound of the desired group VI or VIII transition metalordinarily giving best results.

This primary reduction step can be carried out in a conventionalreaction zone in any suitable manner. The thiosulfate-containingeffluent stream from the preliminary treatment step can be passed intothe reaction zone in either upward, radial or downward flow and thecarbon monoxide stream can be simultaneously introduced into thereaction zone in either countercurrent or concurrent flow relative tothe thiosulfate-containing effluent stream. In particular, a preferredembodiment of this step involves downward flow of the thiosulfate streamwith countercurrent flow of the carbon monoxide stream. It is preferredto utilize suitable means in the reaction zone for effecting intimatecontact between a liquid stream and a gas stream. Suitable contactingmeans include bubble trays, baffles andany of the various packingmaterials known to those skilled in the art. In the preferred case wherea catalyst is utilized in this second step, best results are ordinarilyobtained when the catalyst is maintained within the reaction zone as afixed bed of relatively small particles. These catalyst particlesperform the dual functions of catalyzing the desired reaction and ofpromoting intimate contact between the gas and liquid streams. In thepreferred countercurrent flow mode of operation for this step, a gasstream containing carbon dioxide, unreacted carbon monoxide and somehydrogen sulfide is typically withdrawn from the upper region of thereaction zone. Likewise, an aqueous effluent stream containing thecorresponding sulfide compound is withdrawn from the lower region of thereaction zone. For example, in the case where the input stream to thisprimary reduction step contains ammonium thiosulfate, this aqueouseffluent stream will primarily contain ammonium hydrosulfide with minoramounts of unreacted ammonium thiosulfate, ammonium carbonate andammonium hydroxide.

The reduction conditions utilized in this primary reduction step aretypically relatively more severe than those utilized in the preliminarytreatment step and can be generally characterized as reductionconditions sufficient to effect conversion of thiosulfate to sulfide.The temperature is preferably selected from the range of about to about350C, with best results obtained at a relatively high temperature ofabout to about 350C. It is an essential feature of the present inventionthat the step is conducted under liquid phase conditions; accordingly,the pressure employed must be sufficient to maintain at least a portionof the effluent stream from the first step in the liquid phase.Typically the pressure is selected from the range of about 100 to about3,000 psig., as a function of the reaction temperature in order tomaintain the desired liquid phase condition. Particularly good resultsare obtained at a temperature of about 200C, and a pressure of about 500psig. It is preferred to use a liquid hourly space velocity (defined onthe basis of the liquid volume charge rate of the effluent stream fromthe first step divided by the volume of the reaction zone utilized inthis second step in the case where a catalyst is not utilized and by thevolume of the catalyst bed in the case where a catalyst is used in thissecond step) selected from the range of about 0.25 to about hr., withbest results obtained at about 0.5 to about 3 hr.'. Excellent resultshave been obtained in this step with a LHSV of 1 hr.-'.

In the next step of the present invention the aqueous effluent streamrecovered from the primary reduction step is subjected to a strippingstep designed to liberate hydrogen sulfide therefrom. Although anysuitable stripping gas can be utilized including steam, nitrogen, airand the like, carbon dioxide is particularly preferred, because, it actsto decrease the pH of the solution and form the corresponding carbonatesalt. For instance in the case where the effluent stream from theprimary reduction step contains ammonium hydrosulfide, stripping withcarbon dioxide liberates hydrogen sulfide and produces ammoniumcarbonate.

' decomposition mode of operation is contacted in a conventionaldistillation zone wherein upfiowing vapors are generated by supplyingheat to the bottom of same by means such as a steam coil or conventionalreboiler. Regardless of which mode of operation is employed in thisthird step, an overhead stream containing hydrogen sulfide will beproduced. Likewise, a regenerated aqueous absorbent stream which issubstantially reduced in total sulfur content will be recoveredtherefrom as a bottom stream.

As indicated, this regenerated aqueous absorbent stream is substantiallyreduced in total sulfur content relative to the input rich absorbentstream and usually will contain less than 10 percent of the amount ofsulfur contained in the input absorbent stream. In the case where thecarbon dioxide is utilized in the stripping step as the strippingmedium, this treated water stream will contain substantial amounts ofthe carbonate or bicarbonate salt of the alkaline reagent originallypresent in the input absorbent stream for example in the case where thealkaline reagent is ammonia, the regenerated absorbent stream willcontain ammonium carbonate and/or bicarbonate, and in the case where thealkaline reagent is sodium hydroxide or carbonate the treated waterstream will contain sodium carbonate and/or bicarbonate.

in accordance with the instant process, at least a por- I tion of theregenerated absorbent stream is passed to the scrubbing step. In apreferred embodiment of the instant process, a portion of the hydrogensulfide-containing overhead stream produced in the stripping step ispassed to the preliminary treatment step in order to supply at least aportion of the reducing agent used therein. The remaining portion ofthis hydrogen sulfide-containing stream is then recovered as one of theproduct streams from the instant process. The hydrogen sulfide containedin this product stream can be converted to elemental sulfur by anysuitable oxidation procedure such as a conventional Claus process or tosulfuric acid or used per se.

Having broadly characterized the essential steps comprising the presentprocess, reference is now made to the attached drawing for a detailedexplanation of'a working example of a preferred flow scheme for thepresent invention. The'attached drawing is merely intended as a generalrepresentation of the flow scheme involved with no intention to givedetails about heaters, pumps, valves and the like equipment except wherea knowledge of these devices is essential to an understanding of thepresent invention or would not be self-evident to those skilled in therelevant art.

Referring now to the attached drawing, a flue gas stream enters thesystem via line 1 and is passed into the lower region of a conventionalliquid-gas scrubbing means, zone 2. In this zone it ispassed incountercurrent flow to a descending stream of absorbent solution whichenters the upper region of zone 2 via line 4. The input gas streamcontains about 5% O 12% C0 6% H O, 76.8% N and 0.2% S0 Zone 2 isaconventional-gas liquid contacting zone fitted with conventional meanssuch as baffles, trays, packing material and the like, for effectingintimate contact between an ascending gas stream and a descending liquidstream.

Also introduced into zone 2 is a liquid stream comprising the aqueousabsorbent solution. It enters zone 2 via line 4 and is made up of twoseparate streams, one of which is a regenerated absorbent streamobtained from the regeneration section of the present system and thesecond of which is a major portion of the rich absorbent streamwithdrawn from the lower region of zone 2 via line 4. The alkalinereagent utilized in this absorbent stream is primarily ammoniumcarbonate with a minor amount of ammonium bicarbonate.

According to the mode of operation of zone 2 shown in the drawing, therich absorbent stream is withdrawn from the bottom of zone 2 via line 4and a major portion of this stream is continuously cycled around zone 2in order to allow the concentration of sulfite salts in the Y absorbentstream to build to relatively high levels. This procedure increases thecapacity of the absorbent and minimizes the amount of the absorbentstream that must by cycled through the regeneration section of thesystem. The absorbent stream introduced into scrubber 2 via line 4 willaccordingly contain a substantial amount of sulfite salts along with thealkaline reagent. Ordinarily, zone 2 is operated by monitoring the pH ofthe absorbent stream at the inlet to zone 2 and controlling the amountof the rich absorbent stream diverted to the regeneration section of thesystem at the junction of line 4 and 5 in response to a decrease in pHlevel. The preferred pH range is about four to about seven or more, withbest results ordinarily obtained in the range of about five to seven.With the system operating so that the absorbent stream introduced. intozone 2 via line 4 is maintained ata pH level within this range, the richabsorbent stream withdrawn continuously from zone 2 via line 4 cantypically contain about 1 to about 15 or more wt. percent sulfurprincipally as a mixture of ammonium sulfite and ammonium bisulfite. Inaddition, minor amounts of ammonium sulfate and thiosulfate are formedin zone 2. In the particular case shown in the drawing, the richabsorbent preferably contains about 8 wt. percent sulfur as ammoniumsulfite and bisulfite.

Zone 2 is operated at a temperature of about 50C., a pressure of aboutatmospheric and a gas to absorbent volume ratio of about 500:1. At theseconditions, the treated gas stream withdrawn from the upper region ofzone 2 via line 3 is found to contain less than 5 percent of the Soriginally present in the input gas stream.

As previously explained, the rich absorbent stream withdrawn from thelower region of zone 2 via line 4 is divided into two portions at thejunction of line 5 with line 4. The major portion continues on via line4 and is admixed with regenerated absorbent at the junction of line 15with line 4. The resulting mixture of cycled and regenerated absorbentis then reintroduced into zone 2 via line 4. The minor portion of therich absorbent is passed via line 5 into the first reacting zone, zone6. The amount of rich absorbent passed into zone 6 is ordinarily atleast sufficient to remove the net sulfur input into zone 2 from theinput gas stream in order to line out the concentration of sulfur in theabsorbent stream. In the case under consideration, the amount of theabsorbent withdrawn for regeneration via line 5 will be about 0.1 to 10percent of the rich absorbent stream withdrawn from the bottom of zone2. It is to be noted that during start up of scrubbing zone 2, theinventory of the scrubbing solution needed for initiating operation isintroduced into the system via line 19, and 4. It is also to berecognized that there is a net water make in the regeneration of theabsorbent which ordinarily is removed from the system in the treated gasstream withdrawn from the system via line 3.

The rich absorbent stream introduced into zone 6 via line 5 in theparticular case of interest here contains about 8 wt. percent sulfur asa mixture of ammonium sulfite and ammonium bisulfite. It enters theupper region of zone 6 which, once again, is a conventional liquid-gascontacting zone designed to effect intimate contact with an ascendinggas stream and a descending liquid stream. Also, introduced into zone 6via line 14 is a gas stream containing hydrogen sulfide. During startupof zone 6 sufficient H S is introduced thereto via lines 18 and 4 toinitiate the desired conversion reaction. Thereafter a portion of thehydrogen sulfide-containing gas stream which is produced in thesubsequently described stripping step is passed to zone 6 from zone 13via line 14. In either case the amount of hydrogen sulfide supplied tozone 6 is sufficient to react about 0.5 moles of H 8 per mole ofammonium sulfite plus ammonium bisulfite charged to zone 6. Byconventional means zone 6 is maintained at a temperature of 100C. at apressure of 200 psig. Also the flow rates of the stream into zone 6 areadjusted to provide a residence time of the liquid stream in zone 6 ofabout 0.5 hours.

An overhead gaseous stream, containing unreacted hydrogen sulfide andminor amounts of carbon dioxide and carbon monoxide, is then withdrawnfrom zone 6 via line 7 and vented from the system. Likewise, an aqueouseffluent stream is withdrawn from the lower region of zone 6 via line 8and charged to the second reaction zone, zone 9. This aqueous effluentstream contains ammonium thiosulfate in an amount corresponding to aconversion in zone 6 of greater than 90 percent of the input ammoniumsulfite and bisulfite to ammonium thiosulfate. Furthermore, the amountof undesired ammonium sulfate formed by the reaction in zone 6 is lessthan 3 percent of the input sulfite. Accordingly, the aqueous effluentstream withdrawn from zone 6 via line 8 principally contains ammoniumthiosulfate with minor amounts of unreacted ammonium sulfite andbisulfite.

Zone 9, the second reaction zone, is another liquidgas reaction zonedesigned to effect intimate contact between an ascending gas stream anda descending liquid stream. The aqueous effluent stream from zone 6 isintroduced into the into the upper region of zone 9 by means of line 8.Likewise, a carbon monoxide-rich stream is introduced into the lowerregion of zone 9 by means of line 11. Zone 9 contains a fixed bed of acatalyst comprising 10 to 12 mesh particles of activate carbon having acobalt sulfide component combined therewith in an amount sufficient toresult in a catalyst containing about 5 wt. percent cobalt. The amountof carbon monoxide introduced into the system via line 1 1 correspondsto a carbon monoxide to ammonium thiosulfate mole ratio of about 5.511.The reduction conditions maintained in zone 9 by conventional means area temperature of about 200C., a pressure about 500'psig. and a liquidhourly space velocity of 1 hr.

A sulfide-containing aqueous effluent stream is then withdrawn from thelower region of zone 9 via line 12 and passed to stripping zone 13. Anoverhead gaseous stream is similarly withdrawn from the upper region ofzone 9 via line 10 and passed to the lower region to the strippingzone-13. An analysis of the stream flowing through line 12 indicatesthat 99 percent of the ammonium thiosulfate charged to zone 9 isconverted therein to ammonium hydrosulfide. A similar analysis of theoverhead gas stream indicates that it contains a relatively large amountof carbon dioxide with minor amounts of unreacted carbon monoxide,hydrogen sulfide, ammonia and water. At the junction of line 10 withline 16 additional quantities of CO may be added to the system in orderto increase the efficiency of the stripping operation in zone 13. Inmany cases the amount of CO contained in the overhead stream from zone 9is sufficient for the stripping step and the addition of CO via line 16is not necessary.

in stripping zone 13, the aqueous effluent stream from zone 9 iscountercurrently contacted with an ascending gaseous stream whichessentially comprises the overhead gaseous stream fron zone 9. Zone 13is typically operated at a relatively low temperature and pressure ascompared to zone 9. In fact excellent results are obtained at atemperature of about C. and atmospheric pressure. Although excellentresults can be obtained by stripping at relatively low temperatures, itis, of course, advantageous to use conventional means such as a streamcoil or reboiler to heat the liquid in the bottom portion of zone 13 inorder to further generate up-flowing vapors which aid in the liberationof H 8.

An overhead gaseous stream is then withdrawn from zone 13 via line 14and passed to the junction of line 17 with line 14. The major portion ofthis overhead stream is then withdrawn from the system via line 17. Thegas stream withdrawn via line 17 contains the net sulfide product of thepresent process and, it can be charged to any suitable process for therecovery of sulfur or the manufacture of sulfuric acid if desired; forexample, this stream could be passed to a conventional Claus unit forrecovery of sulfur via an oxidation procedure.

This overhead gaseous stream contains a relatively large amount ofhydrogen sulfide, carbon dioxide and minor amounts of carbon monoxide,ammonia and water. Another portion of this overhead stream is passed vialine 14 to zone 6 in order to supply the hydrogen sulfide reactantnecessary for the conversion of sulfite to thiosulfate in zone 6.

A stream of regenerated absorbent is withdrawn from the lower region ofzone 13 via line 15 and passed back to zone 2 via line 4. Thisregenerated absorbent stream primarily contains a mixture of ammoniumcarbonate and bicarbonate with minor amounts of unreacted ammoniumthiosulfate, unreacted ammonium sulfite, ammonium hydrosulfide andammonium sulfate. The total sulfur content of this regenerated absorbentstream is less than 10 percent of the total sulfur content of the richabsorbent stream withdrawn from the scrubbing section of the system vialine 5. Moreover, the amount of undesired ammonium sulfate formed in theregeneration section of the system (i.e., the section of the systemcomprising zones 6, 9, and 13) is less than 3 percent of the sulfitecharged to the regeneration section via line 5. Thus the scrubbingprocess of the present invention enables the continuous scrubbing of Sfrom the gas stream entering the system via line 1 with continuousregeneration and recirculation of absorbent in a closed-loop manner. Inaddition, the amount of undesired ammonium sulfate formed in theregeneration section of the system is held to extremely low levels.

It is intended to cover by the following examples all changes andmodifications of the above disclosure of the present invention thatwould be self-evident to a man of ordinary skill in the gas treatingart.

I claim as my invention:

1. A process for treating an input gas stream containing SO .in order tocontinuously remove SO, therefrom, said process comprising the steps of:

a. contacting the input gas stream with an aqueous absorbent containingan alkaline reagent at scrubbing conditions selected to result in atreated gas stream containing a reduced amount of S0 and in an effluentwater stream containing a watersoluble sulfite compound;

b. contacting at least a portion of the effluent stream from step (a)with a reducing agent, selected from 2. A process as defined in claim 1wherein the alkaline reagent utilized in the absorbent is ammoniumcarbonate.

3. A process as defined in claim 1 wherein the alkaline reagent utilizedin the absorbent is ammonium hydroxide.

4. A process as defined in claim 1 wherein the alkaline reagent utilizedin the absorbent is an alkali metal hydroxide or carbonate.

5. A process as defined in claim 4 wherein said alkali metal is sodium.

6. A process as defined in claim 4 wherein said alkali metal ispotassium.

7. A process as defined in claim 1 wherein said alkaline reagentutilized in the absorbent stream is an alkaline earth metal hydroxide orcarbonate.

8. A process as defined in claim 1 wherein the thiosulfate productionconditions utilized in step (b) include a temperature of about 20 toabout 150 C. and a pressure sufficient to maintain the'effluent streamfrom step (a) in the liquid phase.

9. A process as defined in claim 1 wherein the reduction conditionsutilized in step (0) include a temperature of about to about 350 C. anda pressure sufficient to maintain the effluent stream from step (b) inthe liquid phase.

10. A process as defined in claim 1 wherein the amount of carbonmonoxide charged to step (c) is sufficient to provide a mole ratio ofcarbon monoxide to thiosulfate of at least 4:1.

I 1. A process as defined in claim 1 wherein the reducing agent utilizedin step (b) is hydrogen sulfide.

12. A process as defined in claim 11 wherein at least a portion of thehydrogen sulfide utilized in step (b) is obtained by passing to step (b)a portion of the hydrogen sulfide stripped in step (d).

13. A process as defined in claim 1 wherein the reducing agent utilizedin step (b) is finely divided sulfur which is used in an amount at leastsufficient to provide a mole ratio of sulfur to sulfite of 1:1

14. A process as defined in claim 1 wherein the reducing agent utilizedin step (b) is a polysulfide compound which is used in an amount atleast sufficient to provide a mole ratio of polysulfide to sulfite of l:6.

15. A process as defined in claim 1 wherein the reducing agent utilizedin step (b) is a water-soluble sulfide compound which is used in anamount at least sufficient to provide a mole ratio of sulfide to sulfiteof 1:2.

16. A process as defined in claim 1 wherein step (c) is performed in thepresence of the catalyst comprising activated carbon.

17. A process as defined in claim 1 wherein step (c) is performed in thepresence of a catalyst comprising a metallic component selected from thegroup consisting of the transition metals of groups VI AND VIII of thePeriodic Table combined with a porous carrier materia. contacting theinput gas stream with an aqueous absorbent containing ammonium carbonateor bicarbonate at scrubbing conditions selected to form a treated gasstream containing a reduced soluble sulfide compound and mixturesthereof, at thiosulfate production conditions selected to result in aneffluent stream containing ammonium ammonium sulfide or hydrosulfide;

d. stripping hydrogen sulfide from the aqueous effluent stream from step(c) with a carbon dioxidecontaining stripping gas to form a regeneratedamount of S and an effluent water stream con- 5 aqueous absorbent streamcontaining ammonium taining ammonium sulfite or bisulfite; bicarbonateor carbonate and an overhead gaseous contacting at least a portion ofthe effluent water Stream containing y g Sulfidfi; and

stream from step (a) with a reducing agent, e e

selected from the group consisting of finely di- 6. passing at least aportion of the resulting vided sulfur, a l lfld compound, awaterregenerated absorbent stream back to step (a).

20. A process as defined in claim 19 wherein the reducing agent utilizedin step (b) is hydrogen sulfide.

21. A process as defined in claim 20 wherein at least a portion of thehydrogen sulfide reactant utilized in step (b) is obtained by passing aportion of the overhead stream produced in step (d) to step (b).

thiosulfate; c. reacting the effluent stream from step (b) with carbonmonoxide at reduction conditions selected to produce an aqueous effluentstream containing

2. A process as defined in claim 1 wherein the alkaline reagent utilizedin the absorbent is ammonium carbonate.
 3. A process as defined in claim1 wherein the alkaline reagent utilized in the absorbent is ammoniumhydroxide.
 4. A process as defined in claim 1 wherein the alkalinereagent utilized in the absorbent is an alkali metal hydroxide orcarbonate.
 5. A process as defined in claim 4 wherein said alkali metalis sodium.
 6. A process as defined in claim 4 wherein said alkali metalis potassium.
 7. A process as defined in claim 1 wherein said alkalinereagent utilized in the absorbent stream is an alkaline earth metalhydroxide or carbonate.
 8. A process as defined in claim 1 wherein thethiosulfate production conditions utilized in step (b) include atemperature of about 20* to about 150* C. and a pressure sufficient tomaintain the effluent stream from step (a) in the liquid phase.
 9. Aprocess as defined in claim 1 wherein the reduction conditions utilizedin step (c) include a temperature of about 100* to about 350* C. and apressure sufficient to maintain the effluent stream from step (b) in theliquid phase.
 10. A process as defined in claim 1 wherein the amount ofcarbon monoxide charged to step (c) is sufficient to provide a moleratio of carbon monoxide to thiosulfate of at least 4:1.
 11. A processas defined in claim 1 wherein the reducing agent utilized in step (b) ishydrogen sulfide.
 12. A process as defined in claim 11 wherein at leasta portion of the hydrogen sulfide utilized in step (b) is obtained bypassing to step (b) a portion of the hydrogen sulfide stripped in step(d).
 13. A process as defined in claim 1 wherein the reducing agentutilized in step (b) is finely divided sulfur which is used in an amountat least sufficient to provide a mole ratio of sulfur to sulfite of 1:1.14. A process as defined in claim 1 wherein the reducing agent utilizedin step (b) is a polysulfide compound which is used in an amount atleast sufficient to provide a mole ratio of polysulfide to sulfite of1:6.
 15. A process as defined in claim 1 wherein the reducing agentutilized in step (b) is a water-soluble sulfide compound which is usedin an amount at least sufficient to provide a mole ratio of sulfide tosulfite of 1:2.
 16. A process as defined in claim 1 wherein step (c) isperformed in the presence of the catalyst comprising activated carbon.17. A process as defined in claim 1 wherein step (c) is performed in thepresence of a catalyst comprising a metallic component selected from thegroup consisting of the transition metals of groups VI AND VIII of thePeriodic Table combined with a porous carrier materIal.
 18. A process asdefined in claim 17 wherein the catalyst is cobalt sulfide combined withactivated carbon or alumina.
 19. A process for treating an input gasstream containing SO2 in order to continuously remove SO2 therefrom,said process comprising the steps of: a. contacting the input gas streamwith an aqueous absorbent containing ammonium carbonate or bicarbonateat scrubbing conditions selected to form a treated gas stream containinga reduced amount of SO2 and an effluent water stream containing ammoniumsulfite or bisulfite; b. contacting at least a portion of the effluentwater stream from step (a) with a reducing agent, selected from thegroup consisting of finely divided sulfur, a polysulfide compound, awater-soluble sulfide compound and mixtures thereof, at thiosulfateproduction conditions selected to result in an effluent streamcontaining ammonium thiosulfate; c. reacting the effluent stream fromstep (b) with carbon monoxide at reduction conditions selected toproduce an aqueous effluent stream containing ammonium sulfide orhydrosulfide; d. stripping hydrogen sulfide from the aqueous effluentstream from step (c) with a carbon dioxide-containing stripping gas toform a regenerated aqueous absorbent stream containing ammoniumbicarbonate or carbonate and an overhead gaseous stream containinghydrogen sulfide; and thereafter, e. passing at least a portion of theresulting regenerated absorbent stream back to step (a).
 20. A processas defined in claim 19 wherein the reducing agent utilized in step (b)is hydrogen sulfide.
 21. A process as defined in claim 20 wherein atleast a portion of the hydrogen sulfide reactant utilized in step (b) isobtained by passing a portion of the overhead stream produced in step(d) to step (b).